Substrate processing method

ABSTRACT

A substrate has a first surface with at least one division line formed thereon and a second surface opposite the first surface. The substrate is processed by applying a pulsed laser beam from the side of the first surface. The substrate is transparent to the pulsed laser beam. The pulsed laser beam is applied at least in a plurality of positions along the at least one division line, a focal point of the pulsed laser beam located at a distance from the first surface in the direction from the first surface towards the second surface, so as to form a plurality of modified regions inside the substrate. Each modified region is entirely within the bulk of the substrate, without any openings open to the first surface or the second surface. Substrate material is removed along the at least one division line where the modified regions are present.

TECHNICAL FIELD

The present invention relates to a method of processing a substrate, having a first surface with at least one division line formed thereon and a second surface opposite the first surface.

TECHNICAL BACKGROUND

In an optical device fabrication process, an optical device layer, e.g., composed of an n-type nitride semiconductor layer and a p-type nitride semiconductor layer, is formed on the front side of a single crystal substrate, such as a sapphire substrate, a silicon carbide (SiC) substrate or a gallium nitride (GaN) substrate. The optical device layer is partitioned by crossing division lines (also referred to as “streets”) to define separate regions where optical devices, such as light emitting diodes (LEDs) and laser diodes, are respectively formed. By providing the optical device layer on the front side of the single crystal substrate, an optical device wafer is formed. The optical device wafer is separated, e.g., cut, along the division lines to divide the separate regions where the optical devices are formed, thereby obtaining the individual optical devices as chips or dies.

As a method of dividing a wafer, such as an optical device wafer, along the division lines, there has been proposed a laser processing method of applying a pulsed laser beam, having a wavelength allowing transmission of the beam through the wafer, to the wafer along the division lines in a condition where a focal point of the pulsed laser beam is located inside the wafer in a subject area to be divided. In this way, a modified layer having a reduced strength is continuously formed inside the wafer along each division line. Subsequently, an external force is applied to the wafer along each division line by using a breaking tool, thereby dividing the wafer into the individual optical devices. Such a method is disclosed in JP-A-3408805.

As another method of dividing a wafer, such as an optical device wafer, along the division lines, it has been proposed to apply a pulsed laser beam to the wafer in a condition where a focal point of the beam is located at a distance from the front side of the wafer in the direction towards the back side thereof, in order to create a plurality of hole regions in the single crystal substrate. Each hole region is composed of an amorphous region and a space in the amorphous region open to the front side of the wafer. Subsequently, an external force is applied to the wafer along each division line by using a breaking tool, thus dividing the wafer into the individual optical devices.

However, when applying the external force to the wafer using the breaking tool in the above-mentioned dividing methods, a shift of the resultant chips or dies relative to each other may occur. Such a die shift not only renders the process of picking up the chips or dies more complicated but also creates the risk of damage to the chips or dies, e.g., if their side surfaces touch each other due to the shift.

Further, the individual chips or dies may not be properly separated from each other by the application of the external force using the breaking tool. For one thing, two or more of the chips or dies may still be, at least partially, connected to each other after the breaking process, so that it is necessary to inspect the wafer after die separation. For another thing, the outer shape of the resultant chips or dies, i.e., the shape of their side surfaces, after separation thereof cannot be controlled with a high degree of accuracy.

The problems referred to above are particularly pronounced for transparent crystal materials which are difficult to process, such as silicon (Si), gallium arsenide (GaAs), gallium nitride (GaN), gallium phosphide (GaP), indium arsenide (InAs), indium phosphide (InP), silicon carbide (SiC), silicon nitride (SiN), lithium tantalate (LT), lithium niobate (LN), sapphire (Al₂O₃), aluminium nitride (AlN), silicon oxide (SiO₂) or the like.

Hence, there remains a need for a method of processing a substrate which allows for the substrate to be processed in an accurate, reliable and efficient manner.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a method of processing a substrate which allows for the substrate to be processed in an accurate, reliable and efficient manner. This goal is achieved by a substrate processing method with the technical features of claim 1. Preferred embodiments of the invention follow from the dependent claims.

The invention provides a method of processing a substrate, having a first surface, e.g., a front surface, with at least one division line formed thereon and a second surface, e.g., a back surface, opposite the first surface. The method comprises applying a pulsed laser beam to the substrate from the side of the first surface. The substrate is made of a material which is transparent to the pulsed laser beam. The pulsed laser beam is applied to the substrate at least in a plurality of positions along the at least one division line, in a condition where a focal point of the pulsed laser beam is located at a distance from the first surface in the direction from the first surface towards the second surface, so as to form a plurality of modified regions inside the substrate, each modified region being arranged entirely within the bulk of the substrate, without forming any openings open to the first surface or the second surface. The method further comprises removing substrate material along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present.

Thus, each modified region is arranged entirely within the bulk of the substrate, so that no openings open to the first surface or the second surface are formed. No opening open to the first surface is formed and no opening open to the second surface is formed. No openings are formed in the first surface and no openings are formed in the second surface.

The modified regions formed by the application of the pulsed laser beam to the substrate are arranged entirely within the bulk of the substrate, i.e., inside the substrate. Hence, the modified regions do not extend all the way to the first surface, i.e., so as to reach the first surface, and the modified regions do not extend all the way to the second surface, i.e., so as to reach the second surface. No open spaces reaching the first surface and no open spaces reaching the second surface are present.

The pulsed laser beam is applied to the substrate at least in a plurality of positions along the at least one division line, i.e., along the extension direction of the at least one division line.

In the method of the invention, the pulsed laser beam is applied to the substrate at least in a plurality of positions along the at least one division line. Hence, the modified regions are formed in the plurality of positions along the at least one division line.

According to the processing method of the invention, the pulsed laser beam is applied to the substrate from the side of the first surface at least in a plurality of positions along the at least one division line, so as to form a plurality of modified regions inside the substrate along the at least one division line. By forming these modified regions, the strength of the substrate in the areas thereof where the modified regions are formed is reduced. Hence, the removal of substrate material along the at least one division line where the plurality of modified regions has been formed is greatly facilitated.

In the method of the invention, substrate material is removed along the at least one division line in the state in which the modified regions, which are arranged entirely within the bulk of the substrate, are present in the substrate.

Since substrate material is removed along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present, no application of an external force by using a breaking tool is necessary in order to divide the substrate.

The substrate can be divided by the removal of substrate material along the at least one division line, thus reliably preventing any shift of the resulting separated parts of the substrate, such as chips or dies, relative to each other and enabling control of the outer shape, i.e., the side surfaces, of these parts with a high degree of accuracy. Moreover, a complete separation of these parts from each other can be reliably and efficiently ensured, so that no subsequent wafer inspection is required.

Hence, the processing method of the invention allows for the substrate to be processed in an accurate, reliable and efficient manner.

A plurality of division lines may be formed on the first surface of the substrate. The method may comprise applying a pulsed laser beam to the substrate from the side of the first surface, at least in a plurality of positions along one or more, preferably all, of the division lines. In this case, a plurality of modified regions is formed inside the substrate at least in a plurality of positions along the one or more, preferably all, of the division lines. Subsequently, substrate material may be removed along the one or more, preferably all, of the division lines where the modified regions arranged entirely within the bulk of the substrate are present.

The pulsed laser beam has a wavelength which allows transmission of the laser beam through the substrate.

The pulsed laser beam may be applied to the substrate at least in a plurality of positions along the at least one division line in such a manner that adjacent ones of the positions do not overlap each other.

The pulsed laser beam may be applied to the substrate at least in a plurality of positions along the at least one division line in such a manner that a distance between adjacent ones of the positions, i.e., a distance between centres of adjacent positions, is within a range of 3 μm to 50 μm, preferably 5 μm to 40 μm and more preferably 8 μm to 30 μm. The plurality of modified regions may be formed in the substrate so that a distance between centres of adjacent modified regions in the extension direction of the at least one division line is in the range of 3 μm to 50 μm, preferably 5 μm to 40 μm and more preferably 8 μm to 30 μm. Particularly preferably, the distance between centres of adjacent modified regions in the extension direction of the at least one division line is in the range of 8 μm to 10 μm.

The modified regions may be equidistantly spaced in the extension direction of the at least one division line. Alternatively, some or all of adjacent or neighbouring modified regions may have different distances from each other in the extension direction of the at least one division line.

The diameters of the modified regions may be substantially constant along the direction from the first surface towards the second surface of the substrate.

The modified regions may have diameters in the range of 1 μm to 30 μm, preferably 2 μm to 20 μm and more preferably 3 μm to 10 μm.

Particularly preferably, the modified regions may have diameters in the range of 2 μm to 3 μm.

The plurality of modified regions may be formed inside the substrate so that adjacent or neighbouring modified regions do not overlap each other. In this way, it can be particularly reliably ensured that the substrate maintains a sufficient degree of strength or robustness for allowing efficient further handling and/or processing thereof, in particular, in the step of removing substrate material along the at least one division line.

Preferably, the distance between outer edges of adjacent or neighbouring modified regions in the width direction of the at least one division line and/or in the extension direction of the at least one division line is at least 1 μm.

The plurality of modified regions may be formed in the substrate so that adjacent or neighbouring modified regions at least partially overlap each other. In some embodiments, adjacent or neighbouring modified regions only overlap each other along a part of the extension of the modified regions along the thickness direction of the substrate. For example, adjacent or neighbouring modified regions may only overlap each other along a part of the extension of the modified regions along the thickness direction of the substrate which is closer to the first surface of the substrate. Adjacent or neighbouring modified regions may be configured so as not to overlap each other along a part of the extension of the modified regions along the thickness direction of the substrate which is closer to the second surface of the substrate.

The substrate may be a single crystal substrate or a glass substrate or a compound substrate, such as a compound semiconductor substrate, e.g., a GaAs substrate. In particularly preferred embodiments, the substrate is a single crystal substrate.

The modified region is a region of the substrate which has been modified by the application of the pulsed laser beam. For example, the modified region may be a region of the substrate in which the structure of the substrate material has been modified by the application of the pulsed laser beam.

The modified region may comprise an amorphous region or a region in which cracks are formed. In particularly preferred embodiments, the modified region comprises an amorphous region.

The modified region may comprise a space, e.g., a cavity, inside the substrate material, the space being surrounded by an amorphous region or a region in which cracks are formed.

The modified region may be composed of a space, e.g., a cavity, inside the substrate material and an amorphous region or a region in which cracks are formed surrounding the space.

The modified region may be an amorphous region or a region in which cracks are formed. In particularly preferred embodiments, the modified region is an amorphous region.

If the modified region comprises or is a region in which cracks are formed, i.e., cracks have been formed, the cracks may be microcracks. The cracks may have dimensions, e.g., lengths and/or widths, in the μm range. For example, the cracks may have widths in the range of 5 μm to 100 μm and/or lengths in the range of 100 μm to 1000 μm.

In some embodiments of the method of the present invention, the substrate is a single crystal substrate, and the method comprises applying a pulsed laser beam to the single crystal substrate from the side of the first surface, at least in a plurality of positions along the at least one division line, so as to form a plurality of modified regions inside the single crystal substrate, each modified region being arranged entirely within the bulk of the substrate, without forming any openings open to the first surface or the second surface, and removing substrate material along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present. The modified regions comprise or are amorphous regions. The amorphous regions render the substrate more fragile in the area where the plurality of modified regions has been formed, thus further facilitating the process of removing substrate material. The pulsed laser beam is applied to the single crystal substrate in a condition where a focal point of the pulsed laser beam is located at a distance from the first surface in the direction from the first surface towards the second surface.

In some embodiments of the method of the present invention, the substrate is a compound substrate, and the method comprises applying a pulsed laser beam to the substrate from the side of the first surface, at least in a plurality of positions along the at least one division line, so as to form a plurality of modified regions inside the substrate, each modified region being arranged entirely within the bulk of the substrate, without forming any openings open to the first surface or the second surface, and removing substrate material along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present. The modified regions comprise or are amorphous regions. The amorphous regions render the substrate more fragile in the area where the plurality of modified regions has been formed, thus further facilitating the process of removing substrate material. The pulsed laser beam is applied to the single crystal substrate in a condition where a focal point of the pulsed laser beam is located at a distance from the first surface in the direction from the first surface towards the second surface.

In some embodiments of the method of the present invention, the substrate is a glass substrate, and the method comprises applying a pulsed laser beam to the glass substrate from the side of the first surface, at least in a plurality of positions along the at least one division line, so as to form a plurality of modified regions inside the glass substrate, each modified region being arranged entirely within the bulk of the substrate, without forming any openings open to the first surface or the second surface, and removing substrate material along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present. The modified regions comprise or are regions in which cracks are formed. The cracks render the substrate more fragile in the area where the plurality of modified regions has been formed, thus further facilitating the process of removing substrate material. The cracks may be microcracks.

The substrate material may be removed by cutting the substrate along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present. The substrate may be cut, for example, by using a mechanical cutting means, such as a blade or a saw, by laser cutting, by plasma cutting, e.g., using a plasma source, etc. Cutting the substrate is a particularly efficient, simple and reliable way of removing the substrate material along the at least one division line.

The substrate material may be mechanically removed along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present. In particular, the substrate material may be mechanically removed by mechanically cutting the substrate along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present. For this purpose, a mechanical cutting means, such as a blade or a saw, may be used.

As has been detailed above, the formation of the plurality of modified regions inside the substrate along the at least one division line reduces the strength of the substrate in the areas where the modified regions are formed. Hence, the mechanical removal of substrate material, in particular, the mechanical cutting of the substrate, along the at least one division line can be performed in a more efficient manner, in particular, with an increased processing speed. For example, for the case of a blade or saw dicing process, the blade or saw dicing speed can be significantly increased.

Moreover, the formation of the plurality of modified regions along the at least one division line can contribute to achieving so-called self-sharpening of a dicing blade or saw, in particular, if an abrasive blade or saw is used in the cutting process. In this case, while performing the removal of substrate material, the blade or saw can be conditioned at the same time. In this way, blade or saw clogging can be reliably avoided. Hence, blade or saw dicing can be carried out with a higher processing load, further increasing the processing rate.

The modified regions are formed so as to extend along only a part of the thickness, in the direction from the first surface towards the second surface, of the substrate. Some or all of the modified regions may be formed so as to extend along 5% or more and 60% or less, preferably 10% or more and 40% or less, and more preferably 15% or more and 30% or less of the thickness of the substrate.

Forming the modified regions with a large extension along the thickness of the substrate is particularly preferable in view of an enhanced service life of a means used for the removal of substrate material, in particular, a blade or a saw. Moreover, in this case, the self-sharpening effect detailed above can be further enhanced.

The amount of the extension of some or all of the modified regions along the thickness of the substrate can be appropriately chosen, e.g., depending on whether it is intended to fully or partially cut the substrate along the thickness thereof.

The amount of extension of the modified regions along the thickness of the substrate and the position of the modified regions along the thickness of the substrate can be accurately controlled, for example, by locating the focal point of the pulsed laser beam at an appropriate distance from the first surface in the direction from the first surface towards the second surface.

In the step of removing substrate material along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present, the substrate material may be removed along only a part of the thickness, in the direction from the first surface towards the second surface, of the substrate. The substrate material may be removed along 30% or more, preferably 40% or more, more preferably 50% or more, even more preferably 60% or more and yet even more preferably 70% or more of the thickness of the substrate.

The substrate material may be removed along the entire thickness of the substrate. In this way, the substrate is divided along the at least one division line by the substrate material removing process.

A substrate material removal width, in the direction substantially perpendicular to the extension direction of the at least one division line, e.g., a cutting width, may be varied in the substrate material removing process. For example, the substrate material may be removed along a part of the substrate thickness with a first removal width and another part, e.g., the remaining part, of the substrate material in the thickness direction of the substrate may be removed with a second removal width. The second removal width may be smaller than the first removal width.

For example, for this purpose, two different cutting means having different widths in the direction substantially perpendicular to the extension direction of the at least one division line can be used.

In some embodiments of the present invention, in each of the plurality of positions along the at least one division line where the pulsed laser beam is applied, plural modified regions, e.g., two or more, three or more, four or more, five or more, or six or more modified layers, may be formed, each modified region being arranged entirely within the bulk of the substrate, and the plural modified regions may be arranged next to one another along the direction from the first surface towards the second surface, i.e., along the thickness direction of the substrate. In this way, plural layers of modified regions can be formed, wherein the plural layers are stacked along the thickness direction of the substrate. Such a stack of layers of modified regions can extend over 30% or more, 40% or more, 50% or more, 60% or more, 70% or more, 80% or more, or 90% or more of the thickness of the substrate.

The method of the present invention may further comprise grinding the second surface of the substrate to adjust the substrate thickness. In this case, it is particularly preferable to form in the substrate plural layers of modified regions which are stacked along the thickness direction of the substrate. In this way, by appropriately arranging one or more of these plural layers in the vicinity of the second surface, the second surface side of the substrate can be reduced in strength, thus allowing for the grinding process to be performed more efficiently, in particular, with a higher grinding speed.

Grinding the second surface of the substrate may be performed before removing the substrate material along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present.

Grinding the second surface of the substrate may be performed after removing the substrate material along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present.

In particular, in the step of removing substrate material along the at least one division line, the substrate material may be removed along only a part of the thickness of the substrate. Subsequently, grinding of the second surface of the substrate may be performed after the substrate material removal along the at least one division line.

The grinding may be carried out in such a way as to reduce the substrate thickness to a thickness that corresponds to the depth at which the substrate material has been removed along the at least one division line, for example, to the cutting depth of a cutting process. In this case, the substrate material which had not been reached by the substrate material removing process along the at least one division line is removed in the grinding step, so that the substrate is divided along the at least one division line by the grinding process.

Grinding of the second surface of the substrate may thus be performed along a remaining part of the thickness of the substrate, in which no substrate material has been removed, so as to divide the substrate along the at least one division line.

By dividing the substrate in the grinding step in the manner detailed above, the substrate can be processed in a particularly reliable, accurate and efficient manner. Specifically, the step of removing substrate material along the at least one division line is performed on the substrate before grinding, i.e., before a reduction in thickness thereof. Hence, any deformation of the substrate during material removal, e.g., during cutting, along the at least one division line, such as substrate warpage or the like, can be reliably avoided. Further, the stress applied to the substrate during substrate material removal along the at least one division line is significantly reduced, allowing for chips or dies with an increased die strength to be obtained. Any damage to the resulting chips or dies, such as the formation of cracks or back side chipping, can be prevented.

Moreover, since the substrate material is removed along the at least one division line only along a part of the substrate thickness, the efficiency, in particular, the processing speed, of the substrate material removal process is enhanced. Also, the service life of a means, e.g., a cutting means, used for the substrate material removing step is extended.

For the case of removing the substrate material along only a part of the thickness of the substrate and subsequently grinding the second surface of the substrate in the manner described above, so as to divide the substrate along the at least one division line, it is particularly preferable to form plural layers of modified regions inside the substrate, wherein the plural layers are stacked along the thickness direction of the substrate. In this way, the efficiencies of both the step of substrate material removal along the at least one division line and the grinding step can be significantly enhanced.

In the step of removing substrate material along the at least one division line, the substrate material may be removed along the entire extension, in the direction from the first surface towards the second surface, of the modified regions, or along only a part of this extension. The substrate material may be removed along 30% or more, preferably 40% or more, more preferably 50% or more, even more preferably 60% or more and yet even more preferably 70% or more of the extension of the modified regions.

The at least one division line formed on the first surface of the substrate may have a width in a direction substantially perpendicular to the extension direction of the at least one division line.

The width of the at least one division line may be in the range of 30 μm to 200 μm, preferably 30 μm to 150 μm and more preferably 30 μm to 100 μm.

The pulsed laser beam may be applied to the substrate from the side of the first surface also in a plurality of positions along the width direction of the at least one division line.

A plurality of modified regions may be formed within the width of the at least one division line.

Adjacent or neighbouring modified regions may be equidistantly spaced in the width direction of the at least one division line. Alternatively, some or all of adjacent or neighbouring modified regions may have different distances from each other in the width direction of the at least one division line. The modified regions may be substantially randomly arranged in the extension direction and/or the width direction of the at least one division line.

The distances between adjacent modified regions in the width direction of the at least one division line, i.e., between centres of adjacent modified regions, may be in the range of 3 μm to 50 μm, preferably 5 μm to 40 μm and more preferably 8 μm to 30 μm.

The pulsed laser beam may be applied also in a plurality of positions along the width direction of the at least one division line, so as to form within the width of the at least one division line a plurality of rows of modified regions, each row extending along the extension direction of the at least one division line. The rows may be arranged adjacent to each other in the width direction of the at least one division line. The rows may be equidistantly spaced in the width direction of the at least one division line or some or all of adjacent rows may have different distances from each other in the width direction of the at least one division line.

The distance between adjacent rows of modified regions in the width direction of the at least one division line, i.e., between centres of the modified regions of the adjacent rows, may be in the range of 3 μm to 50 μm, preferably 5 μm to 40 μm and more preferably 8 μm to 30 μm. The number of rows may be in the range of 2 to 20, preferably 4 to 18, more preferably 5 to 15 and even more preferably 8 to 12.

Alternatively, a single row of modified regions may be formed within the width of the at least one division line. For example, the modified regions may have diameters of 17.5 μm or more, preferably 35 μm or more, and more preferably 70 μm or more.

By forming within the width of the division line a plurality of rows of modified regions arranged adjacent to each other in the width direction of the division line as detailed above, the process of removing substrate material along the division line, in particular, by using a cutting process, e.g., a mechanical cutting process, can be rendered even more efficient.

Further, a wide variation of means for removing substrate material along the at least one division line, e.g., a wide variation of mechanical cutting means, such as blades or saws, e.g., having different cutting widths, can be used. Moreover, for example, cutting blades or saws with a reduced hardness or strength can be employed, due to the reduced strength of the area of the substrate where the modified regions are formed, allowing for the costs of the cutting means or equipment to be reduced. Also, the service life of the cutting means or equipment can be extended.

The rows of modified regions may be formed in such a manner that the distance between adjacent rows is larger at or closer to the centre of the at least one division line in the width direction of the at least one division line than at positions arranged further away from the centre of the division line in the width direction of the at least one division line, e.g., in edge areas or side areas of the division line. In particular, the rows of modified regions may be present only in these edge areas or side areas of the at least one division line.

By arranging the rows of modified regions in such a manner that the distance between adjacent rows is larger at the centre of the division line than at positions arranged away from the centre of the division line, the process of forming the modified regions can be rendered more efficient, since the number of modified regions can be reduced.

The substrate material may be removed along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present by mechanically cutting the substrate using a cutting means.

A width, in the direction substantially perpendicular to the extension direction of the at least one division line, of an area of the substrate in which the row or rows of modified regions have been formed may be smaller than a width, in the direction substantially perpendicular to the extension direction of the at least one division line, of the cutting means. The area of the substrate in which the rows of modified regions have been formed is the area of the substrate between the two outermost rows of modified regions in the width direction of the at least one division line.

In this way, it can be reliably ensured that, in the process of mechanically cutting the substrate, the entire area of the substrate in which the modified regions have been formed can be removed. Hence, a particularly high quality of the outer or side surfaces of the substrate parts, such as chips or dies, obtained in the process of dividing the substrate can be achieved.

The width, in the direction substantially perpendicular to the extension direction of the at least one division line, of the area of the substrate in which the row or rows of modified regions have been formed may be larger than the width, in the direction substantially perpendicular to the extension direction of the at least one division line, of the cutting means. In this way, the cutting process can be performed in a particularly efficient and quick manner. Modified regions remaining on the separated parts of the substrate after the cutting process may be subsequently removed, e.g., by polishing the outer or side surfaces of the resulting substrate parts, e.g., chips or dies.

The width of the area of the substrate in which the row or rows of modified regions have been formed may be in a range of approximately 80% to 120%, preferably 90% to 110% and more preferably 95% to 105% of the width of the cutting means. In this way, it can be ensured that the cutting process can be carried out in an efficient manner, while obtaining separated substrate parts, such as chips or dies, with a good quality of the outer or side surfaces thereof.

The width of the area of the substrate in which the row or rows of modified regions have been formed may be in a range of approximately 80% to 120%, preferably 80% to 110%, more preferably 80% to 105%, even more preferably 90% to 105%, and yet even more preferably 95% to 105% of the width of the at least one division line.

A row or rows of modified regions arranged closer to the centre of the at least one division line, in the width direction of the at least one division line, may be formed with a pulsed laser beam having a higher power than a pulsed laser beam used for forming a row or rows of modified regions arranged further away from the centre of the at least one division line, in the width direction of the at least one division line. In this way, the efficiency of the process of removing substrate material along the at least one division line, in particular, by cutting, e.g., mechanical cutting, can be further enhanced.

The substrate is made of a material which is transparent to the pulsed laser beam. Thus, the plurality of modified regions is formed in the substrate by the application of a pulsed laser beam having a wavelength that allows transmission of the laser beam through the substrate.

For example, if the substrate is a silicon (Si) substrate, the pulsed laser beam may have a wavelength of 1.0 μm or more.

The pulsed laser beam may have a pulse width, for example, in the range of 1 ns to 300 ns.

The substrate may be, for example, a semiconductor substrate, a glass substrate, a sapphire (Al₂O₃) substrate, a ceramic substrate, such as an alumina ceramic substrate, a quartz substrate, a zirconia substrate, a PZT (lead zirconate titanate) substrate, a polycarbonate substrate, an optical crystal material substrate or the like.

In particular, the substrate may be, for example, a silicon (Si) substrate, a gallium arsenide (GaAs) substrate, a gallium nitride (GaN) substrate, a gallium phosphide (GaP) substrate, an indium arsenide (InAs) substrate, an indium phosphide (InP) substrate, a silicon carbide (SiC) substrate, a silicon nitride (SiN) substrate, a lithium tantalate (LT) substrate, a lithium niobate (LN) substrate, a sapphire (Al₂O₃) substrate, an aluminium nitride (AlN) substrate, a silicon oxide (SiO₂) substrate or the like.

The substrate may be made of a single material or of a combination of different materials, e.g., two or more of the above-identified materials.

The pulsed laser beam may be focused using a focusing lens. A numerical aperture (NA) of the focusing lens may be set so that the value obtained by dividing the numerical aperture of the focusing lens by the refractive index (n) of the substrate is in the range of 0.2 to 0.85. In this way, the modified regions can be formed in a particularly reliable and efficient manner.

BRIEF DESCRIPTION OF THE DRAWINGS

Hereinafter, non-limiting examples of the invention are explained with reference to the drawings, in which:

FIG. 1 shows an optical device wafer as a substrate to be processed by the method of the invention, wherein FIG. 1(a) is a perspective view of the wafer, and FIG. 1(b) is an enlarged cross-sectional view of the encircled region A in FIG. 1(a);

FIG. 2 is a perspective view showing a condition in which the optical device wafer of FIG. 1(a) is attached to an adhesive tape supported by an annular frame;

FIG. 3 is a perspective view of a part of a laser processing apparatus for applying a pulsed laser beam to the optical device wafer of FIG. 1(a);

FIG. 4 is a side view for illustrating a step of forming a plurality of modified regions inside the optical device wafer of FIG. 1(a) according to an embodiment of the method of the present invention;

FIGS. 5 (a) to 5(d) are schematic cross-sectional views illustrating the formation of a modified region inside the optical device wafer of FIG. 1(a);

FIG. 6 shows a process of dividing the optical device wafer according to an embodiment of the processing method of the present invention, wherein FIGS. 6(a) and 6(b) are cross-sectional views illustrating a step of removing substrate material along a division line, and FIG. 6(c) is a cross-sectional view illustrating a grinding step;

FIG. 7 shows steps of removing substrate material along a division line for two different embodiments of the present invention, wherein FIGS. 7(a) and 7(b) are cross-sectional views illustrating the substrate material removing step for one embodiment, and FIGS. 7(c) and 7(d) are cross-sectional views illustrating the substrate material removing step for another embodiment;

FIGS. 8(a) and 8(b) illustrate steps of removing substrate material along a division line for further different embodiments of the present invention; and

FIGS. 9(a) and 9(b) show cross-sectional views of examples of arrangements of rows of modified regions for different embodiments of the method of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. The preferred embodiments relate to methods of processing an optical device wafer as a substrate.

The optical device wafer may have a thickness before grinding in the μm range, preferably in the range of 200 μm to 1500 μm.

FIG. 1(a) is a perspective view of an optical device wafer 2 as a substrate to be processed by the processing method of the present invention. The optical device wafer 2 is a single crystal substrate.

In other embodiments, the substrate to be processed by the processing method of the present invention may be a glass substrate or a compound substrate, such as a compound semiconductor substrate, e.g., a GaAs substrate.

The optical device wafer 2 shown in FIG. 1(a) is substantially composed of a sapphire substrate with a thickness of, for example, 300 μm. A plurality of optical devices 21, such as light emitting diodes (LEDs) and laser diodes, are formed on a front side 2 a, i.e., a first surface, of the sapphire substrate. The optical devices 21 are provided on the front side 2 a of the sapphire substrate in a grid or matrix arrangement. The optical devices 21 are separated by a plurality of crossing division lines 22 formed on the front side 2 a of the sapphire substrate, i.e., on the front side of the optical device wafer 2.

In the following, a preferred embodiment of the method of the present invention for processing the optical device wafer 2 as the substrate will be described with reference to FIGS. 2 to 6(c).

First, a wafer supporting step is performed in such a manner that the optical device wafer 2 is attached to an adhesive tape, such as a dicing tape, supported by an annular frame. Specifically, as is shown in FIG. 2, an adhesive tape 30, e.g., a dicing tape, is supported at a peripheral portion thereof by an annular frame 3, so as to close an inner opening of the annular frame 3 by the adhesive tape 30. A back side 2 b, i.e., a second surface, of the optical device wafer 2 is attached to the adhesive tape 30. Accordingly, the front side 2 a of the optical device wafer 2 attached to the adhesive tape 30 is oriented upwards, as is shown in FIG. 2.

FIG. 3 shows a part of a laser processing apparatus 4 for performing laser processing along the division lines 22 on the optical device wafer 2 after carrying out the wafer supporting step described above. As is shown in FIG. 3, the laser processing apparatus 4 includes a chuck table 41 for holding a workpiece, in particular, the optical device wafer 2, a laser beam applying means 42 for applying a laser beam to the workpiece held on the chuck table 41, and an imaging means 43 for imaging the workpiece held on the chuck table 41. The chuck table 41 has an upper surface as a holding surface for holding the workpiece thereon under suction. The chuck table 41 is movable by a feeding means (not shown) in a feeding direction which is indicated in FIG. 3 by an arrow X. Further, the chuck table 41 is movable by an indexing means (not shown) in an indexing direction which is indicated in FIG. 3 by an arrow Y.

The laser beam applying means 42 includes a cylindrical casing 421 extending in a substantially horizontal direction. The casing 421 contains a pulsed laser beam oscillating means (not shown) including a pulsed laser oscillator and a repetition frequency setting means. Further, the laser beam applying means 42 includes a focusing means 422 mounted on a front end of the casing 421. The focusing means 422 comprises a focusing lens 422 a for focusing a pulsed laser beam oscillated by the pulsed laser beam oscillating means.

The numerical aperture (NA) of the focusing lens 422 a of the focusing means 422 may be set so that the value obtained by dividing the numerical aperture of the focusing lens 422 a by the refractive index (n) of the single crystal substrate is within the range of 0.2 to 0.85.

The laser beam applying means 42 further includes a focal position adjusting means (not shown) for adjusting the focal position of the pulsed laser beam to be focused by the focusing lens 422 a of the focusing means 422.

The imaging means 43 is mounted on a front end portion of the casing 421 of the laser beam applying means 42. The imaging means 43 includes an ordinary imaging device (not shown), such as a CCD, for imaging the workpiece by using visible light, an infrared light applying means (not shown) for applying infrared light to the workpiece, an optical system (not shown) for capturing the infrared light applied to the workpiece by the infrared light applying means, and an infrared imaging device (not shown), such as an infrared CCD, for outputting an electrical signal corresponding to the infrared light captured by the optical system. An image signal output from the imaging means 43 is transmitted to a control means (not shown).

When performing laser processing along the division lines 22 of the optical device wafer 2 by using the laser processing apparatus 4, a positioning step is performed in such a manner that the focusing lens 422 a of the focusing means 422 and the single crystal substrate, i.e., the optical device wafer 2, are positioned relative to each other in the direction along the optical axis of the focusing lens 422 a so that the focal point of the pulsed laser beam is located at a desired position in the direction along the thickness of the optical device wafer 2, i.e., at a desired distance from the front side 2 a, i.e., the first surface, in the direction from the front side 2 a towards the back side 2 b, i.e., the second surface.

When performing the processing method according to the current embodiment of the present invention, the optical device wafer 2 attached to the adhesive tape 30 is first placed on the chuck table 41 of the laser processing apparatus 4 shown in FIG. 3 in the condition where the adhesive tape 30 is in contact with the upper surface of the chuck table 41 (see FIG. 3). Subsequently, a suction means (not shown) is operated to hold the optical device wafer 2 through the adhesive tape 30 on the chuck table 41 under suction (wafer holding step). Accordingly, the front side 2 a of the optical device wafer 2 held on the chuck table 41 is oriented upward. Although, for the purpose of better presentability, the annular frame 3 supporting the adhesive tape 30 is not shown in FIG. 3, the annular frame 3 is held by a frame holding means, such as clamps or the like, provided on the chuck table 41. Subsequently, the chuck table 41 holding the optical device wafer 2 under suction is moved to a position directly below the imaging means 43 by operating the feeding means.

In the condition where the chuck table 41 is positioned directly below the imaging means 43, an alignment operation is performed by the imaging means 43 and the control means (not shown) in order to detect a subject area of the optical device wafer 2 to be laser processed. Specifically, the imaging means 43 and the control means perform image processing, such as pattern matching, in order to align the division lines 22 extending in a first direction on the optical device wafer 2 with the focusing means 422 of the laser beam applying means 42. In this way, alignment of a laser beam applying position is performed (alignment step). This alignment step is performed in a similar manner also for all the other division lines 22 extending in a second direction perpendicular to the first direction on the optical device wafer 2.

After performing the alignment step detailed above for all of the division lines 22 on the front side 2 a of the optical device wafer 2, the chuck table 41 is moved to a laser beam applying area where the focusing means 422 of the laser beam applying means 42 is located, as is shown in FIG. 4. One end (the left end in FIG. 4) of a predetermined division line 22 extending in the first direction is positioned directly below the focusing means 422. Further, the focal position adjusting means (not shown) is operated so as to move the focusing means 422 in the direction along the optical axis of the focusing lens 422 a so that the focal point P of a pulsed laser beam LB to be focused by the focusing lens 422 is located at a desired distance from the front side 2 a of the optical device wafer 2 in the direction from the front side 2 a towards the back side 2 b thereof, i.e., in the thickness direction of the optical device wafer 2 (positioning step).

In this preferred embodiment, the focal point P of the pulsed laser beam LB is located inside the optical device wafer 2 at a position near the front side 2 a, i.e., the upper surface, of the optical device wafer 2 to which the pulsed laser beam LB is applied. For example, the focal point P may be located at a distance from the front side 2 a in the range of 5 μm to 10 μm.

After performing the positioning step described above, a modified region forming step is performed in such a manner that the laser beam applying means 42 is operated to apply the pulsed laser LB from the focusing means 422 to the optical device wafer 2, thereby forming a modified region inside the optical device wafer 2, the modified region being arranged entirely within the bulk of the wafer 2, without forming any openings open to the first surface 2 a or the second surface 2 b of the wafer 2 (see also FIG. 5(d)).

Specifically, the pulsed laser beam LB, which has a wavelength that allows transmission of the laser beam LB through the sapphire substrate constituting the optical device wafer 2, is applied to the optical device wafer 2 by the focusing means 422, and the chuck table 41 is moved at a predetermined feed speed in the direction indicated by an arrow X1 in FIG. 4 (modified region forming step). When the other end (right end in FIG. 4) of the predetermined division line 22 reaches the position directly below the focusing means 422, the application of the pulsed laser beam LB is stopped and the movement of the chuck table 41 is also stopped.

By performing the modified region forming step detailed above along the predetermined division line 22, a plurality of modified regions 23 (see FIG. 5(d)) is formed in the optical device wafer 2 along the division line 22, wherein each modified region 23 is arranged entirely within the bulk of the wafer 2. Each modified region 23 is composed of a space 231, e.g., a cavity, inside the wafer material and an amorphous region 232 surrounding the space 231, as is schematically shown in FIG. 5(d) and will be further detailed below.

The modified regions 23 may be formed along the division line 22 at predetermined, equidistant intervals in the extension direction of the division line 22. For example, the distance between adjacent modified regions 23 in the extension direction of the division line 22 may be in the range of 8 μm to 30 μm, e.g., approximately 16 μm (=(work feed speed: 800 mm/second)/(repetition frequency: 50 kHz)).

In this embodiment, the amorphous regions 232 of adjacent modified regions 23 are formed so as not to overlap each other (in this regard, see FIGS. 9(a) and (b)). Specifically, the distance between adjacent modified regions 23 is chosen so as to be slightly larger than the outer diameter of the amorphous regions 232. The amorphous regions of adjacent or neighbouring modified regions 23 are thus disconnected from each other.

In other embodiments, the substrate may be, for example, a glass substrate and the modified regions may comprise or be regions in which cracks are formed in the glass substrate. The cracks formed in the glass substrate may be microcracks.

It is sufficient to apply the pulsed laser beam LB once for the formation of each modified region 23, so that the productivity can be greatly enhanced. Furthermore, no debris is scattered in the modified region forming step, so that a degradation of the quality of the resulting devices can be reliably prevented.

The formation of a modified region 23 inside the optical device wafer 2 is illustrated in FIGS. 5(a) to 5(d). The substrate of the wafer 2 is made of a material which is transparent to the pulsed laser beam LB, namely sapphire. Thus, the modified region 23 is formed in the wafer 2 by the application of the pulsed laser beam LB having a wavelength that allows transmission of the laser beam LB through the wafer 2. For example, the pulsed laser beam LB may have a wavelength in the infrared range, e.g., 1064 nm.

The pulsed laser beam LB is applied to the wafer 2 from the side of the first surface 2 a in a condition where the focal point P of the pulsed laser beam LB is located at a distance from the first surface 2 a in the direction from the first surface 2 a towards the second surface 2 b (see FIG. 5(a)). Due to the application of the pulsed laser beam LB, the wafer material is locally heated up in the region inside the wafer 2 where the focal point P is arranged. The heated up region of the wafer 2 at the initial stage of the laser beam application is schematically indicated by a circle in FIG. 5 (b).

As the application of the pulsed laser beam LB continues, the heated up region grows or expands in the direction towards the first surface 2 a, as is indicated by an arrow in FIG. 5(c). When the laser beam application is stopped, the heated up wafer material cools down, resulting in the formation of the modified region 23 which is composed of the space 231 inside the wafer 2 and the amorphous region 232 fully surrounding the space 231 (see FIG. 5(d)). As is shown in FIG. 5(d), the modified region 23 is arranged entirely within the bulk of the wafer 2, without any openings open to the first surface 2 a or the second surface 2 b being formed.

The division lines 22 have a width w in a direction substantially perpendicular to the extension direction thereof, as is schematically shown in FIG. 1(b). The modified region forming step detailed above is performed along the predetermined division line 22 two or more times, while slightly shifting the optical device wafer 2 relative to the laser beam applying means 42 in the indexing direction (indicated by the arrow Y in FIG. 3), so as to apply the pulsed laser beam LB also in a plurality of positions along the width direction of the division line 22. In this way, a plurality of modified regions 23 is formed also along the width direction of the division line 22. The modified regions 23 may be arranged with different distances between adjacent modified regions 23 in the extension direction and/or the width direction of the division line 22, as is schematically shown in FIG. 1(b).

A plurality of rows of modified regions 23 may be formed within the width w of the division line 22, each row extending along the extension direction of the division line 22, wherein the rows are arranged adjacent to each other in the width direction of the division line 22. The rows of modified regions 23 may be equidistantly arranged in the width direction of the division line 22, as is schematically shown in FIGS. 6 and 7. Alternatively, the distances between adjacent rows of modified regions 23 in the width direction of the division line 22 may vary. For example, the distances between adjacent rows of modified regions 23 may be larger at or closer to the centre of the division line 22 than at positions further away from the centre, i.e., at the sides or edges in the width direction of the division line 22, as is shown in FIG. 8(a).

Further, in each of the plurality of positions along the division line 22 where the pulsed laser beam LB is applied, plural modified regions 23 may be formed, each modified region 23 being arranged entirely within the bulk of the substrate, and the plural modified regions 23 may be arranged next to one another along the direction from the front side 2 a towards the back side 2 b, i.e., along the thickness direction of the wafer 2. In this way, plural layers of modified regions 23 can be formed, wherein the plural layers are stacked along the thickness direction of the wafer 2.

In the embodiments shown in FIGS. 6 to 8, plural rows of modified regions 23 are arranged next to each other in the width direction of the division line 22, and plural layers of these plural rows are stacked along the thickness direction of the wafer 2.

In other embodiments, a single row of modified regions 23 may be formed within the width w of the division line 22.

After performing the modified region forming step a plurality of times along the predetermined division line 22 as detailed above, the chuck table 41 is moved in the indexing direction (indicated by the arrow Y in FIG. 3) by the pitch of the division lines 22 extending in the first direction on the optical device wafer 2 (indexing step). Subsequently, the modified region forming step is performed a plurality of times in the same manner as described above along the next division line 22 extending in the first direction. In this way, the modified region forming step is performed a plurality of times along all of the division lines 22 extending in the first direction. Thereafter, the chuck table 41 is rotated by 90°, in order to perform the modified region forming step a plurality of times in the same manner as detailed above along all of the other division lines 22 extending in the second direction perpendicular to the first direction.

The modified region forming step may be performed using a pulsed laser beam with a wavelength in the range of 300 nm to 3000 nm, a pulse width of 0.5 ps to 500 ps, an average power of 0.2 W to 10.0 W and a repetition frequency of 10 kHz to 100 kHz. The work feed speed with which the optical device wafer 2 is moved relative to the laser beam applying means 42 in the modified region forming step may be in the range of 500 mm/second to 1000 mm/second.

If a semiconductor substrate is used as the substrate, e.g., the single crystal substrate, to be processed by the method of the present invention, a modified region 23 can be formed in a particularly efficient and reliable manner if the wavelength of the pulsed laser beam LB is set to a value which is two or more times the wavelength (reduced wavelength) corresponding to the band gap of the semiconductor substrate.

After performing the modified region forming step in the manner detailed above, a step of dividing the optical device wafer 2 is carried out, as will be described in detail below with reference to FIGS. 6(a) to (c).

FIG. 6(a) shows a cross-sectional view of a part of the optical device wafer 2 including a division line 22. As has been indicated above, a plurality of rows of modified regions 23, namely six rows of modified regions 23, are formed within the width of the division line 22, each row extending along the extension direction of the division line 22. The rows of modified regions 23 are equidistantly arranged adjacent to each other in the width direction of the division line 22. As is further shown in FIG. 6(a), three layers of six rows of modified regions 23 are stacked along the thickness direction of the wafer 2.

While only one division line 22 is shown in FIG. 6(a), the remaining division lines 22 are provided with three layers of six rows of modified regions 23 in the same manner as shown in this figure.

In the process of dividing the optical device wafer 2, substrate material is removed along the division line 22 first, using a cutting means 6, such as a rotating blade or a saw, as is schematically shown in FIGS. 6(a) and (b). As is shown in these figures, a width, in the direction substantially perpendicular to the extension direction of the division line 22, of an area of the optical device wafer 2 in which the rows of modified regions 23 have been formed is substantially the same as a width, in the direction substantially perpendicular to the extension direction of the division line 22, of the cutting means 6.

The cutting means 6 is moved towards the front side 2 a of the optical device wafer 2 and made to cut into the area of the wafer 2 in which the rows of modified regions 23 have been formed, as is indicated by the arrows in FIGS. 6(a) and (b). As is shown in FIG. 6(c), in the cutting step, the substrate material is removed along only a part of the thickness, in the direction from the front side 2 a towards the back side 2 b, of the optical device wafer 2. For example, the substrate material may be removed along approximately 50% of the thickness of the optical device wafer 2 in the cutting step.

The cutting step is performed in the manner detailed above for all of the division lines 22 formed on the front side 2 a of the optical device wafer 2. Subsequently, the back side 2 b of the optical device wafer 2 is ground using a grinding apparatus (not shown), as is illustrated in FIG. 6(c). The grinding apparatus may include a chuck table (not shown) for holding a workpiece and a grinding means (not shown) for grinding the workpiece held on the chuck table. The chuck table may have an upper surface as a holding surface for holding the workpiece thereon under suction. The grinding means may include a spindle housing (not shown), a rotating spindle (not shown) rotatably supported to the spindle housing and adapted to be rotated by a driving mechanism (not shown), a mounter (not shown) fixed to the lower end of the rotating spindle and a grinding tool 8 (see FIG. 6(c)) mounted on the lower surface of the mounter. The grinding tool 8 may comprise a circular base 81 and abrasive elements 82 mounted on the lower surface of the circular base 81.

Grinding of the back side 2 b of the optical device wafer 2 is performed by holding the wafer 2 on the chuck table (not shown) of the grinding apparatus so that the front side 2 a of the wafer 2 is in contact with the upper surface of the chuck table. Hence, the back side 2 b of the wafer 2 is oriented upwards. Subsequently, the chuck table with the optical device wafer 2 held thereon is rotated around an axis perpendicular to the plane of the optical device wafer 2 and the grinding tool 8 is rotated around an axis perpendicular to the plane of the circular base 81. While rotating the chuck table and the grinding tool 8 in this manner, the abrasive elements 82 of the grinding tool 8 are brought into contact with the back side 2 b of the wafer 2, thus grinding the back side 2 b. Grinding is performed along a remaining part of the thickness of the optical device wafer 2, in which no substrate material has been removed in the cutting step, so as to divide the wafer 2 along the division lines 22.

Dividing the optical device wafer 2 in this manner allows for individual chips or dies (not shown) with a high die strength and high quality side surfaces to be obtained in a particularly accurate, reliable and efficient manner.

In particular, since plural layers of modified regions 23 are stacked along the thickness direction of the wafer 2, the back side 2 b of the wafer 2 has been reduced in strength. Hence, the grinding process can be performed particularly efficiently, in particular, with an especially high grinding speed.

In the following, further preferred embodiments of the present invention will be described with reference to FIGS. 7 and 8.

These embodiments differ from the embodiments detailed above with reference to FIGS. 1 to 6 in the arrangement of the modified regions 23 and/or in the details of the step of removing substrate material along the division lines.

The substrate material removing step, i.e., the cutting step, illustrated in FIGS. 7(a) and (b) differs from the substrate material removing step illustrated in FIGS. 6(a) and (b) mainly in that the width of the area of the optical device wafer 2 in which the rows of modified regions 23 have been formed is smaller than the width of the cutting means 6, as is shown in FIG. 7(a). Further, as is shown in FIG. 7(b), the substrate material is removed along the entire thickness of the optical device wafer 2, i.e., the wafer 2 is cut by the cutting means 6 along its entire thickness.

Performing the substrate material removing step in this manner offers the benefit that the modified regions 23 can be prevented from remaining on the side surfaces 2 c of the resulting chips or dies in a particularly reliable manner (see FIG. 7(b)). Hence, high quality chips or dies can be obtained in a simple way.

The substrate material removing step, i.e., the cutting step, illustrated in FIGS. 7(c) and (d) differs from the substrate material removing step illustrated in FIGS. 6(a) and (b) mainly in that the width of the area of the optical device wafer 2 in which the rows of modified regions 23 have been formed is larger than the width of the cutting means 6, as is shown in FIG. 7(c). Further, the substrate material is removed along the entire thickness of the optical device wafer 2, i.e., the wafer 2 is cut along its entire thickness by the cutting means 6, as is shown in FIG. 7(d).

In this way, the substrate material removing step can be particularly efficiently carried out, since it can be reliably ensured that substantially the entire width of the cutting area in which the cutting means 6 comes into contact with the optical device wafer 2 has been reduced in strength by the formation of the modified regions 23.

Modified regions 23 remaining on the side surfaces 2 c of the resulting chips or dies (see FIG. 7(d)) can be removed in an additional grinding or polishing step, if desired.

As is schematically shown in FIGS. 6(a) and (b), 7(a), (c) and (d) and 8(a) and (b), the modified regions 23 are arranged entirely within the bulk of the wafer 2, without forming any openings open to the first surface 2 a or the second surface 2 b. The modified regions 23 are thus formed so as to extend along only a part of the thickness of the optical device wafer 2. Hence, the formation of the modified regions 23 can be performed in a particularly efficient manner.

As is illustrated in FIG. 6(b), the substrate material may be removed along only a part of the thickness of the optical device wafer 2, e.g., the optical device wafer 2 may be cut along only a part of its thickness. In this case, the optical device wafer 2 may be divided, for example, by grinding the back side 2 b thereof in the manner described in detail above.

Alternatively, as is schematically shown in FIGS. 7(b) and (d), the substrate material may be removed along the entire thickness of the optical device wafer 2, e.g., the wafer 2 may be cut along its entire thickness.

A distance between adjacent rows of modified regions 23 in the width direction of the division line 22 may be larger for rows of modified regions 23 arranged closer to the centre of the division line 22 than for rows of modified regions 23 arranged further away from the centre of the division line 22, as is schematically shown in FIG. 8(a). In this case, fewer rows of modified regions 23 have to be formed, thus enhancing the efficiency of the modified region forming step. Further, the smaller distance between adjacent rows of modified regions 23 away from the centre of the division line 22 ensures that damage to the side surfaces of the resulting chips or dies in the cutting process, such as chipping or cracking, can be reduced.

Also in the embodiment shown in FIG. 8(a), three layers of six rows of modified regions 23 are stacked along the thickness direction of the wafer 2.

The step of removing substrate material along the division line 22 may be performed with different material removal widths. For example, in a first material removing step, substrate material may be removed with a first width and, in a second material removing step, substrate material may be removed with a second width. The second removal width may be smaller than the first removal width.

In particular, as is shown in FIG. 8(b), in the substrate material removing step, the optical device wafer 2 may be first cut with a first cutting means 6 along a part of its thickness. The width of the first cutting means 6 may be substantially the same as the width of the area of the optical device wafer 2 in which the rows of modified regions 23 have been formed. Subsequently, a remaining part of the area in which the rows of modified regions 23 have been formed may be cut using a second cutting means 6′ (shown by a dotted line in FIG. 8(b)) with a width that is smaller than that of the first cutting means 6.

A row or rows of modified regions 23 arranged closer to the centre of the division line 22, in the width direction of the division line 22 (see, for example, FIGS. 6(a) and (b), FIGS. 7(a) and (c) and FIGS. 8(a) and (b)), may be formed with a pulsed laser beam LB having a higher power than a pulsed laser beam LB used for forming a row or rows of modified regions 23 arranged further away from the centre of the division line 22, in the width direction of the division line 22.

FIGS. 9(a) and 9(b) show examples of arrangements of rows of modified regions 23 for different embodiments of the method of the present invention. As is shown in these drawings, the modified regions 23 do not overlap with each other.

FIG. 9(a) shows an example in which seven rows of modified regions 23 are arranged next to each other along the width direction of the division line 22. The division line 22 has a width w1 of approximately 100 μm. A width w2, in the direction substantially perpendicular to the extension direction of the division line 22, of the area of the substrate 2, in which the rows of modified regions 23 have been formed, is approximately 48 μm.

A distance w3 between centres of adjacent modified regions 23 in the extension direction of the division line 22 is in the range of 8 μm to 10 μm. A distance w4 between adjacent rows of modified regions 23 in the width direction of the division line 22, i.e., between centres of the modified regions 23 of the adjacent rows, is in the range of 8 μm to 10 μm. The modified regions 23 have diameters d in the range of 2 μm to 3 μm.

A distance w5 between outer edges of adjacent modified regions 23 in the width direction of the division line 22 is 1 μm or more. A distance w6 between outer edges of adjacent modified regions 23 in the extension direction of the division line 22 is 1 μm or more.

Substrate material may be removed along the division line 22 where the modified regions 23 are formed, e.g., by using a cutting means (not shown), such as a blade or a saw. Particularly preferably, the cutting means may have a width, in the direction substantially perpendicular to the extension direction of the division line 22, which is slightly larger than the width w2 of the area of the substrate 2, in which the rows of modified regions 23 have been formed. For example, the cutting means may have a width of approximately 50 μm.

The arrangement of rows of modified regions 23 shown in FIG. 9(b) differs from that shown in FIG. 9(a) only in the number of rows of modified regions 23, the width w1 of the division line 22 and the width w2, in the direction substantially perpendicular to the extension direction of the division line 22, of the area of the substrate 2, in which the rows of modified regions 23 have been formed.

Specifically, FIG. 9(b) shows an example in which three rows of modified regions 23 are arranged next to each other along the width direction of the division line 22. The division line 22 shown in FIG. 9(b) has a width w1 of approximately 50 μm. The width w2 of the area of the substrate 2, in which the rows of modified regions 23 have been formed, is approximately 22 μm.

Substrate material may be removed along the division line 22 shown in FIG. 9(b) where the modified regions 23 are formed, e.g., by using a cutting means (not shown), such as a blade or a saw. Particularly preferably, the cutting means may have a width, in the direction substantially perpendicular to the extension direction of the division line 22, which is slightly larger than the width w2 of the area of the substrate 2, in which the rows of modified regions 23 have been formed. For example, the cutting means may have a width of approximately 25 μm.

In other embodiments, a single row of modified regions 23 may be formed within the width of the division line 22.

In the embodiments shown in FIGS. 9(a) and (b), a single layer of the modified regions 23 can be formed in the wafer 2. Alternatively, plural layers of the modified regions 23 may be stacked along the thickness direction of the wafer 2, for example, in the manner shown in FIGS. 6 to 8. 

The invention claimed is:
 1. A method of processing a substrate, having a first surface with at least one division line formed thereon and a second surface opposite the first surface the method comprising: applying a pulsed laser beam to the substrate from the side of the first surface, wherein the substrate is made of a material which is transparent to the pulsed laser beam and the pulsed laser beam is applied to the substrate at least in a plurality of positions along the at least one division line, in a condition where a focal point of the pulsed laser beam is located at a distance from the first surface in the direction from the first surface towards the second surface, so as to form a plurality of modified regions inside the substrate along the at least one division line, the plurality of modified regions being formed inside the substrate so that adjacent modified regions do not overlap each other in the extension direction of the at least one division line, each modified region being arranged entirely within the bulk of the substrate, without forming any openings open to the first surface or the second surface; and removing substrate material along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present.
 2. The method according to claim 1, wherein the substrate is a single crystal substrate or a glass substrate or a compound substrate.
 3. The method according to claim 1, wherein the modified regions comprise amorphous regions or regions in which cracks are formed, or the modified regions are amorphous regions or regions in which cracks are formed.
 4. The method according to claim 1, wherein the substrate material is removed by cutting the substrate along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present.
 5. The method according to claim 1, wherein the substrate material is mechanically removed along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present, by mechanically cutting the substrate along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present.
 6. The method according to claim 1, further comprising grinding the second surface of the substrate to adjust the substrate thickness.
 7. The method according to claim 6, wherein grinding the second surface of the substrate is performed after removing the substrate material along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present.
 8. The method according to claim 7, wherein the substrate material is removed along only a part of the thickness, in the direction from the first surface towards the second surface, of the substrate, and grinding the second surface of the substrate is performed along a remaining part of the thickness of the substrate, in which no substrate material has been removed, so as to divide the substrate along the at least one division line.
 9. The method according to claim 1, wherein, in each of the plurality of positions along the at least one division line where the pulsed laser beam is applied, plural modified regions are formed, each modified region being arranged entirely within the bulk of the substrate, and the plural modified regions being arranged next to one another along the direction from the first surface towards the second surface.
 10. The method according to claim 1, wherein the substrate material is removed along the entire extension, in the direction from the first surface towards the second surface, of the modified regions.
 11. The method according to claim 1, wherein the at least one division line has a width in a direction perpendicular to the extension direction of the at least one division line, and the method further comprises applying the pulsed laser beam also in a plurality of positions along the width direction of the at least one division line, so as to form within the width of the division line a plurality of rows of modified regions, each row extending along the extension direction of the at least one division line, wherein the rows are arranged adjacent to each other in the width direction of the at least one division line.
 12. The method according to claim 11, wherein the substrate material is removed along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present by mechanically cutting the substrate, using a cutting means, and a width, in the direction perpendicular to the extension direction of the at least one division line, of an area of the substrate, in which the rows of modified regions have been formed, is in a range of 90% to 110% of a width, in the direction perpendicular to the extension direction of the at least one division line, of the cutting means.
 13. The method according to claim 12, wherein a row or rows of modified regions arranged closer to the center of the at least one division line, in the width direction of the at least one division line, is or are formed with a pulsed laser beam having a higher power than a pulsed laser beam used for forming a row or rows of modified regions arranged further away from the center of the at least one division line, in the width direction of the at least one division line.
 14. The method according to claim 1, wherein the substrate material is mechanically removed along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present , only by mechanically cutting the substrate along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present. 